1. Field of the Invention
The present invention relates to a fuel cell system, and to a method for discharging a reaction gas from a fuel cell.
Priority is claimed on Japanese Patent Application No. 2003-382619, filed Nov. 12, 2003, the content of which is incorporated herein by reference.
2. Description of the Related Art
As a type of fuel cell to be installed in a fuel cell powered vehicle, a fuel cell is known in which electrical power is generated through chemical reactions of reaction gases. Among such fuel cells, a type of fuel cell is known which includes an anode and a cathode that together sandwich a solid polymer electrolyte membrane therebetween, and in which a fuel gas (e.g., a hydrogen gas) is supplied to the anode, and an oxidizing gas (e.g., air containing oxygen) is supplied to the cathode so that chemical energy produced in an oxidation and reduction reaction of these reaction gases is directly output as electrical energy.
In this type of fuel cell, water is formed at the cathode area by the power generating reaction, and a portion of the formed water permeates the solid polymer electrolyte membrane so as to diffuse into the anode area. In addition, a small portion of nitrogen contained in air supplied to the cathode permeates the solid polymer electrolyte membrane so as to diffuse into the anode area, and is mixed with the hydrogen gas. The power generation operation in the fuel cell may become unstable due to impurities such as the formed water and nitrogen.
In particular, in a circulation type fuel cell in which an unreacted hydrogen gas that is discharged from fuel cell is recycled and supplied to the fuel cell again after being mixed with a fresh hydrogen gas in order to increase fuel efficiency, concentration of the above-mentioned impurities at the anode area tends to gradually increase.
In order to solve such a problem of the above type of fuel cell, concentration of the impurities contained in the hydrogen gas is reduced by discharging the hydrogen gas containing the above-mentioned impurities from a hydrogen circulation path periodically or depending on the state of power generation in the fuel cell (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2000-243417).
According to a known process, when discharging of the impurities is demanded, e.g., when the operation time of power generation of the fuel cell reaches a predetermined time, a demanded discharge amount of the hydrogen gas containing impurities or a demanded discharge time of the hydrogen gas is determined depending on the present operation state of the fuel cell.
In such a process, because a flexible control operation is not carried out even when the demanded discharge amount of the hydrogen gas or the demanded discharge time of the hydrogen gas is changed due to change in the operation state of the fuel cell during an operation for discharging the impurities, various problems may be encountered.
For example, when the demanded discharged amount (or the demanded discharge time) after the operation state is changed is less the demanded discharge amount (or the demanded discharge time) that has been determined at the beginning of discharge of the impurities, an excessive amount of hydrogen gas containing impurities is discharged after the operation state is changed. As a result, the amount of consumed hydrogen is substantially increased, and the power generation efficiency is decreased. In addition, in the case in which the fuel cell system includes a dilution system that dilutes the hydrogen gas discharged from the hydrogen circulation path by mixing with a cathode off-gas discharged from the cathode area, it is difficult to make hydrogen concentration after dilution.
On the other hand, when the demanded discharge amount (or the demanded discharge time) after the operation state is changed is greater than the demanded discharge amount (or the demanded discharge time) that has been determined at the beginning of discharge of the impurities, an insufficient amount of hydrogen gas containing impurities may be discharged after the operation state is changed. As a result, stability of the power generation operation of the fuel cell may not be recovered.